Simple Explanation

A way to rate how noisy or quiet a room is based on the background sound.

Concise Technical Definition

A set of standardized curves used to evaluate the background noise level in indoor environments, particularly for measuring the acceptability of HVAC system noise.

Layman-Friendly Analogy

Like a "quietness scorecard" that tells you if a room is calm enough for sleeping, working, or watching movies.

Industry Usage Summary

NC curves are widely used in architecture, acoustical consulting, and building design to ensure acceptable background noise levels in offices, studios, classrooms, and home theaters.

Engineering Shortcut

Standard curves for rating background noise in occupied spaces (focus on HVAC).

Full Technical Explanation

NC (Noise Criterion) curves are a series of standardized measurement curves developed to quantify and limit background noise in indoor occupied spaces. Introduced in the 1950s by Leo Beranek, the NC system is primarily used to evaluate the acoustic impact of HVAC systems, though it applies broadly to any ambient noise. Measurements are taken across octave bands (typically 63 Hz to 8 kHz) and plotted against predefined NC curves; the highest tangent point determines the room’s NC rating. Lower values (e.g., NC-20) indicate quieter environments suitable for bedrooms or recording studios, while higher values (e.g., NC-40) may be acceptable for open offices or lobbies. NC curves are part of building standards and are considered in LEED, WELL, and ASHRAE guidelines for acoustic comfort.